1. Field of the Invention
The present invention relates to a reciprocating internal combustion engine, and particularly to a reciprocating internal combustion engine having a geometric compression ratio of 13.0 or greater.
2. Description of the Background Art
There has been proposed a reciprocating internal combustion engine designed such that valve timings of an intake valve and an exhaust valve are adjusted to have a valve overlap period for allowing both the intake and exhaust valves to be opened at a timing when a piston is located around its top dead center position (see, for example, JP 2006-283631A). Based on setting up the valve overlap period, combusted-gas scavenging efficiency and intake-air charging efficiency can be improved.
In reciprocating internal combustion engines, as a technique for improving fuel economy, it has also been proposed to set a geometric compression ratio at a higher value, e.g., 13 or greater, to improve thermal efficiency. However, given that an engine displacement is constant, as the geometric compression ratio is set at a higher value, a volume of a combustion chamber becomes smaller, which restrict a gas-flow directed in an intake-to-exhaust direction in the combustion chamber. Therefore, if the valve overlap period is set up in an engine having a geometric compression ratio set at a high value, intake and exhaust capacities are not enhanced in proportion to an increase in valve lift amount, resulting in failing to improve combusted-gas scavenging efficiency and intake-air charging efficiency, in some cases. Although a volume of the combustion chamber is increased to remedy this problem, the geometric compression ratio will be reduced. Thus, there is a need for a combustion chamber having a configuration which allows a geometric compression ratio to be set at a higher value while suppressing a resistance to a gas-flow directed in an intake-to-exhaust direction in the combustion chamber.